1. Field of the Invention
This invention relates to a hydraulic control system in an automatic transmission for a vehicle, and more particularly to improvements in a hydraulic control system in an automatic transmission for a vehicle, wherein, in order to achieve an up shift from a specific gear stage to another gear stage in a drive range (D.range), there are provided a frictionally engaging device for the normal engagement arranged in series with a one-way clutch and a frictionally engaging device for the coasting during engine braking action arranged in parallel to the frictionally engaging device for the normal engagement.
2. Description of the Prior Art
There has been widely known an automatic transmission for a vehicle, constructed such that a gear stage changing mechanism and a plurality of frictionally engaging devices (clutches and brakes) are provided, and a hydraulic control system is operated to selectively change the engagement of the frictionally engaging devices, to thereby achieve any one of a plurality of gear stages.
An automatic transmission, wherein planetary gear units are utilized as the gear stage changing mechanism, can achieve gear stages of low, high, and reverse by an arrangement wherein an input shaft, an output shaft and the planetary gear units are provided coaxially with one another, and clutches, brakes and one-way clutches are combined. Accordingly, an automatic transmission of this type has been combined with a torque converter, to thereby be commonly used as the automatic transmission for the vehicle.
In the case of the automatic transmission for a vehicle, in the gear stage in the low speed range, the rotational speed to be transmitted to the output shaft is low, however, the torque to be transmitted is high. As a consequence, clutches, brakes and the like for rotating and braking the gears of the planetary gear units are subjected to severe durability standards. When the engine has a large piston displacement, the throttle opening is large etc. and the torque to be transmitted is further increased, the requirement for durability becomes even more severe. Particularly, a frictionally engaging device for the 2nd gear stage, which is to be newly engaged at the 1st-2nd gear shaft through a one-way clutch, is frequently used during running of the vehicle, whereby this frictionally engaging device is referred to one subjected to the most severe conditions.
On the other hand, as the FF (Front engine--Front wheel drive) has been used more widely in the driving system of a vehicle, there have occurred trends of high rpms and high output in the engines mounted on the vehicle. An automatic transmission in a vehicle of the FF type presents a disadvantage in that it is extremely difficult to increase the number of plates in a multi-plate clutch and in a multi-plate brake of frictionally engaging devices, due to the restriction placed upon the total length of the automatic transmission. In view of the above-described disadvantage, in order to improve the durability of the frictionally engaging devices in the automatic transmission, there has been proposed an automatic transmission, wherein, at the time of an up shift, e.g. from the 1st gear stage to the 2nd gear stage in the drive range (D.range), in addition to a frictionally engaging device for the 2nd gear stage, which is normally engaged, a frictionally engaging device for coasting, which is engaged during engine braking in a so-called 2nd range running, is used as an auxiliary one.
However, with the conventional automatic transmission of this type, the number of socks occurring during shifts become high, because such is a frictionally engaging device, which has been originally designed to have a capacity capable of transmitting solely the torque, and moreover, is used simultaneously with the frictionally engaging device for the coasting mode.